1. Field of the Invention
The present invention relates to a light-emitting element driving circuit.
2. Description of the Related Art
Some electronic devices such as a mobile phone include a display device for displaying time, characters and the like with a plurality of LEDs (Light Emitting Diodes) arranged in a matrix manner. One LED in the display device in which the LEDs are arranged in the matrix manner corresponds to a dot, which is a minimum display unit. Thus, in order to produce desired display on the display device, brightness of each LED needs to be set. FIG. 7 illustrates an example of an LED driving circuit 900 for driving a dot matrix LED 800 in which LEDs are arranged in a matrix with 7 rows and 17 columns (See Japanese Patent Laid-Open Publication No. 2003-158300, for example). The LED driving circuit 900 is a circuit for driving the dot matrix LED 800 by a dynamic drive method on the basis of a command and data inputted from a microcomputer 810, and includes a gradation data storage unit 910, an IF (Interface) circuit 911, a controller 912, a scan line driver 913, and a data line driver 914. The gradation data storage unit 910 is a memory circuit for storing gradation data indicating brightness of the LED for each LED in the dot matrix LED 800. The IF circuit 911 transfers to the controller 912 the gradation data outputted from the microcomputer 810, a driving command for instructing the start of the LED driving and the like. The controller 912 sequentially stores in the gradation data storage unit 910 the inputted gradation data corresponding to each LED. Also, when a drive command is inputted, the controller 912 controls the gradation data storage unit 910, the scan line driver 913, and the data line driver 914 so that the driving of the dot matrix LED 800 is started. Specifically, the controller 912 controls the scan line driver 913 so that scan lines 1A to 7A of the dot matrix LED 800 are sequentially selected on the basis of the drive command. Moreover, the controller 912 sequentially reads the gradation data in the gradation data storage unit 910 to be output to the data line driver 914 so that each of the LEDs connected to the selected scan line is driven on the basis of the corresponding gradation data. As a result, the data line driver 914 outputs a driving current according to the gradation data to each of data lines 1B to 17B. Therefore, the dot matrix LED 800 emits light at the brightness according to the gradation data of the gradation data storage unit 910.
If the LED driving circuit 900 fades out predetermined display of the dot matrix LED 800, for example, the LED driving 900 is required to change the gradation data for each dot so that the entire brightness of the dot matrix LED 800 is gradually dimmed. As such, the controller 912 sequentially stores in the gradation data storage unit 910 the gradation data from the microcomputer 810 corresponding to each LED. Then, the data line driver 914 outputs a driving current according to the gradation data stored in the gradation data storage unit 910 to each of the data lines 1B to 17B. Therefore, if the predetermined display is faded out while the dot matrix LED 800 is being driven by the dynamic drive method, the LEDs whose brightness has been updated and those not updated might be mixed in the 17 LEDs connected to the same scan line of the dot matrix LED 800. As a result, brightness of the LEDs in the dot matrix LED 800 might be varied, which is a problem.